1. Field of the Invention
This invention relates to a surface roughness measuring instrument and specifically to a surface roughness measuring instrument used to measure the surface roughness of spheres, solid or hollow cylinders and the like.
2. Prior Art Statement
The increasing complexity of advanced industrial products and trends toward smaller, lighter, thinner and more compact products have created demand for higher precision machining technologies. Surface roughness measuring instruments for measuring the surface working precision of various objects are employed to this end.
When measuring the surface roughness of a solid or hollow cylinder or other body of revolution in the circumferential direction, the use of an instrument in which the body of revolution is supported on a pair of rollers arranged in parallel and measured while being rotated by one of the rollers is common knowledge.
However, various problems remain with these body-of-rotation surface roughness measuring instruments described above.
Specifically, if such a device for measuring the surface roughness of a solid or hollow cylinder or other body of revolution in the circumferential direction were to be used to support and rotate a sphere, the sphere would roll back and forth along the rollers, making measurement impossible. To remedy this situation, there have been previous attempts to provide a continuous groove around the circumference of one of the rollers and fit the sphere into the groove so that it is supported in a fixed position on the roller without impeding its natural rotation.
For example, the rotary drive mechanism 1 of a surface roughness measuring instrument shown in FIG. 18 is provided with a pair of parallel rollers 3 and 4 arranged in a frame 2 and a rotary drive motor 5 which drives these rollers to rotate. A continuous groove 6 which has a V-shaped cross section is formed around the circumference of roller 4. Thus when a sphere 7 is placed on the two rollers 3 and 4, the sphere 7 fits into the groove 6 and its back and forth motion along rollers 3 and 4 is controlled so that it rotates in a fixed position on rollers 3 and 4.
With this sort of rotary drive mechanism, the groove allows the sphere to be supported by the two rollers at three points, so it will not roll back and forth along the rollers. However, since only one of the pair of rollers is driven, in the case of a sphere of low weight or in other cases, friction from the other roller acts to prevent the smooth rotation of the sphere. In contrast, if both rollers are synchronously driven at equal speeds, the sphere will rotate smoothly.
However, in the case of the rotary drive mechanism 1 of FIG. 18 above in which a groove 6 is provided to support the sphere 7, if one attempts to drive both rollers 3 and 4, the angular velocities at the three points of contact between the sphere 7 and rollers 3 and 4 must all be made equal.
Specifically, when the sphere 7 is fit into a groove 6 on roller 4 as shown in FIG. 19, the radius of rotation R.sub.1 of the point of contact with respect to axis of rotation A.sub.2 is equal to the radius R.sub.0 of roller 4, but the radius of rotation r.sub.1 of the sphere 7 about its axis of rotation A.sub.1 is shorter than the radius r.sub.0 of the sphere 7, so the sphere 7 will rotate with a greater angular velocity than the angular velocity given it by roller 4. Furthermore, if the sphere 7 has a relatively small diameter as shown in FIG. 20, the sphere 7 and roller 4 will come into contact on the conical surfaces 8 and 9 which make up the groove 6, so not only is the radius of rotation r.sub.1 of the point of contact on the sphere 7 side shorter than the radius R.sub.0 of roller 4, but also the radius of rotation R.sub.1 of the point of contact on the roller 4 side is also shorter than the radius R.sub.0 of roller 4, so the difference in angular velocity provided by rollers 3 and 4 becomes even more complex.
Therefore, when attempting to drive both rollers 3 and 4 in the rotary drive mechanism 1 of FIG. 18, a problem occurs in that the sphere 7 cannot rotate stably.
To solve this problem, as in the rotary drive mechanism 1 shown in FIG. 21, similar grooves 6 can be provided on both rollers 3 and 4, so that the change in radii of rotation of the point of contact of sphere 7 with rollers 3 and 4 is equal for both rollers and the angular velocity provided by rollers 3 and 4 to the sphere 7 will always be equal.
However, in this mechanism 1, not only must identical grooves 6 be formed on each of the rollers 3 and 4, but the sphere 7 would be supported by four points, two on each of the grooves 6, so in order for accurate contact to be made at each point, precise axial orientation of rollers 3 and 4 to each other is necessary, so the complexity of manufacture and high costs becomes a problem.
Furthermore, the nose piece 10 of surface roughness measuring instruments used when measuring the surface roughness of a solid or hollow cylinder or other body of revolution in the circumferential direction conventionally has the same structure, shown in FIG. 22, as those used when measuring the surface roughness of a normal flat surface. Specifically, a tip section 13 containing skids 11 and 12 which touch the object being measured near the location being measured is secured to a protector 14 which covers and protects the stylus of the surface roughness measuring instrument and has a fixed position relative to mounting section 15 for mounting onto the surface roughness measuring instrument.
An alternate nose piece 10 for surface roughness measuring instruments for spheres has been proposed in which, as shown in FIG. 23, the skids 11 and 12 which touch the object being measured, namely a sphere 7, are provided with surfaces oriented nearly tangentially to the sphere, thus assuring that the contact is nearly tangential. However, with such a nose piece 10 for surface roughness measuring instruments, the tip section 13 which holds skids 11 and 12, as in the nose piece 10 shown in FIG. 22 above, is fixed relative to the mounting section 15 for mounting onto the surface roughness measuring instrument.
Yet as shown in FIG. 24, measurement of the surface roughness in the circumferential direction of the object being measured, namely a body of revolution 16, is normally carried out by supporting the body of revolution 16 on two rollers whose axes are mutually parallel. After a tip contact section 18, on the tip of a detector arm 17 of the surface roughness measuring instrument to which a nose piece 10 is attached, is placed in contact with the body of revolution 16, measurement is carried out as roller 4 is rotated to drive the body of revolution 16 so that it rotates in its circumferential direction.
However, in comparison to the numerical level of surface roughness to be found on the body of revolution 16, the cross-sectional shapes of the rollers which drive the body of revolution 16 are typically by no means perfect circles, and moreover the rollers are not driven and supported so that they will rotate in a perfectly circular fashion. Similarly, the cross-sectional shape of the body of revolution 16 itself cannot be called a perfect circle in comparison to the numerical level of surface roughness which is typically measured. Therefore, with the skids 11 and 12 each touching body of revolution 16 with approximately the same pressure, rocking of the axis of rotation 19 of the roller will occur for the reasons described above so that the roller will rotate about a tilted axis 20, and the body of revolution 16 will be driven so that it rotates about an axis of rotation 21 nearly parallel to axis of rotation 20. Thus the skids 11 and 12 which had initially protruded in a direction parallel to axis of rotation 19 would be in a state in which skid 11 is lifted away from the body of revolution 16 and only skid 12 remains in contact with the body of revolution 16. During measurement of surface roughness in this state, specifically the measurement of the up and down motion of tip contact section 18, there is a problem that added to the difference in the up and down position due to surface roughness of body of revolution 16 is a certain amount of error due to the position height of the point of contact of tip contact section 18 with respect to the point of contact of skid 12 on body of revolution 16. Particularly since this error is in fact of a numerical order greater than the amount of surface roughness to be found, the problem is extremely severe.
In the case of measuring the surface roughness of a sphere 7 by supporting the sphere 7 on rollers whose axes are mutually parallel and driving the sphere 7 to rotate by rotating a roller, since the relative position of the mounting section 15 for mounting onto the surface roughness measuring instrument and the tip section 13 which supports skids 11 and 12 is also similarly fixed, the same problem described above occurs.